Faculty Bibliography

Vitamin D and intact parathyroid hormone (iPTH) concentrations differ between individuals of African and European descent and may play a role in observed racial differences in bone mineral density (BMD). These findings suggest that mapping by admixture linkage disequilibrium (MALD) may be informative for identifying genetic variants contributing to these ethnic disparities. Admixture mapping was performed for serum 25-hydroxyvitamin D, 1,25-dihydroxyvitamin D, vitamin D-binding protein (VDBP), bioavailable vitamin D, and iPTH concentrations and computed tomography measured thoracic and lumbar vertebral volumetric BMD in 552 unrelated African Americans with type 2 diabetes from the African American-Diabetes Heart Study. Genotyping was performed using a custom Illumina ancestry informative marker (AIM) panel. For each AIM, the probability of inheriting 0, 1, or 2 copies of a European-derived allele was determined. Non-parametric linkage analysis was performed by testing for association between each AIM using these probabilities among phenotypes, accounting for global ancestry, age, and gender. Fine-mapping of MALD peaks was facilitated by genome-wide association study (GWAS) data. VDBP levels were significantly linked in proximity to the protein coding locus (rs7689609, LOD=11.05). Two loci exhibited significant linkage signals for 1,25-dihydroxyvitamin D on 13q21.2 (rs1622710, LOD=3.20) and 12q13.2 (rs11171526, LOD=3.10). iPTH was significantly linked on 9q31.3 (rs7854368, LOD=3.14). Fine-mapping with GWAS data revealed significant known (rs7041 with VDBP, P=1.38×10(-82)) and novel (rs12741813 and rs10863774 with VDBP, P<6.43×10(-5)) loci with plausible biological roles. Admixture mapping in combination with fine-mapping has focused efforts to identify loci contributing to ethnic differences in vitamin D-related traits.

BACKGROUND:Apolipoprotein L1 gene (APOL1) G1 and G2 renal-risk variants, common in populations with recent African ancestry, are strongly associated with non-diabetic nephropathy, end-stage kidney disease, and shorter allograft survival in deceased-donor kidneys (autosomal recessive inheritance). Circulating APOL1 protein is synthesized primarily in the liver and hydrodynamic gene delivery of APOL1 G1 and G2 risk variants has caused hepatic necrosis in a murine model. METHODS:To evaluate the impact of these variants in liver transplantation, this multicenter study investigated the association of APOL1 G1 and G2 alleles in deceased African American liver donors with allograft survival. Transplant recipients were followed for liver allograft survival using data from the Scientific Registry of Transplant Recipients. RESULTS:Of the 639 liver donors evaluated, 247 had no APOL1 risk allele, 300 had 1 risk allele, and 92 had 2 risk alleles. Graft failure assessed at 15 days, 6 months, 1 year and total was not significantly associated with donor APOL1 genotype (p-values = 0.25, 0.19, 0.67 and 0.89, respectively). CONCLUSIONS:In contrast to kidney transplantation, deceased-donor APOL1 G1 and G2 risk variants do not significantly impact outcomes in liver transplantation.

Autoimmune thyroid diseases (AITD) and Type 1 diabetes (T1D) frequently occur in the same individual pointing to a strong shared genetic susceptibility. Indeed, the co-occurrence of T1D and AITD in the same individual is classified as a variant of the autoimmune polyglandular syndrome type 3 (designated APS3v). Our aim was to identify new genes and mechanisms causing the co-occurrence of T1D + AITD (APS3v) in the same individual using a genome-wide approach. For our discovery set we analyzed 346 Caucasian APS3v patients and 727 gender and ethnicity matched healthy controls. Genotyping was performed using the Illumina Human660W-Quad.v1. The replication set included 185 APS3v patients and 340 controls. Association analyses were performed using the PLINK program, and pathway analyses were performed using the MAGENTA software. We identified multiple signals within the HLA region and conditioning studies suggested that a few of them contributed independently to the strong association of the HLA locus with APS3v. Outside the HLA region, variants in GPR103, a gene not suggested by previous studies of APS3v, T1D, or AITD, showed genome-wide significance (p < 5 × 10(-8)). In addition, a locus on 1p13 containing the PTPN22 gene showed genome-wide significant associations. Pathway analysis demonstrated that cell cycle, B-cell development, CD40, and CTLA-4 signaling were the major pathways contributing to the pathogenesis of APS3v. These findings suggest that complex mechanisms involving T-cell and B-cell pathways are involved in the strong genetic association between AITD and T1D.

Apolipoprotein L1 gene (APOL1) G1 and G2 variants are strongly associated with progressive nondiabetic nephropathy in populations with recent African ancestry. Selection for these variants occurred as a result of protection from human African trypanosomiasis (HAT). Resequencing of this region in 10 genetically and geographically distinct African populations residing in HAT endemic regions identified eight single nucleotide polymorphisms (SNPs) in strong linkage disequilibrium and comprising a novel G3 haplotype. To determine whether the APOL1 G3 haplotype was associated with nephropathy, G1, G2, and G3 SNPs and 70 ancestry informative markers spanning the genome were genotyped in 937 African Americans with nondiabetic ESRD, 965 African Americans with type 2 diabetes-associated ESRD, and 1029 non-nephropathy controls. In analyses adjusting for age, sex, APOL1 G1/G2 risk (recessive), and global African ancestry, the G3 haplotype was not significantly associated with ESRD (P=0.05 for nondiabetic ESRD, P=0.57 for diabetes-associated ESRD, and P=0.27 for all-cause ESRD). We conclude that variation in APOL1 G3 makes a nominal, if any, contribution to ESRD in African Americans; G1 and G2 variants explain the vast majority of nondiabetic nephropathy susceptibility.

OBJECTIVE:Albuminuria and reduced kidney function are associated with cognitive impairment. Relationships between nephropathy and cerebral structural changes remain poorly defined, particularly in African Americans (AAs), a population at higher risk for both cognitive impairment and diabetes than European Americans. We examined the relationship between urine albumin:creatinine ratio (UACR), estimated glomerular filtration rate (eGFR), and cerebral MRI volumes in 263 AAs with type 2 diabetes. RESEARCH DESIGN AND METHODS/METHODS:Cross-sectional associations between renal parameters and white matter (WM), gray matter (GM), hippocampal, and WM lesion (WML) volumes were assessed using generalized linear models adjusted for age, education, sex, BMI, hemoglobin A1c (HbA1c) level, and hypertension. RESULTS:Participants had a mean (SD) age of 60.2 years (9.7 years), and 62.7% were female. Mean diabetes duration was 14.3 years (8.9 years), HbA1c level was 8.2% (2.2%; 66 mmol/mol), eGFR was 86.0 mL/min/1.73 m(2) (23.2 mL/min/1.73 m(2)), and UACR was 155.8 mg/g (542.1 mg/g; median 8.1 mg/g). Those with chronic kidney disease (CKD) (eGFR <60 mL/min/1.73 m(2) or UACR >30 mg/g) had smaller GM and higher WML volumes. Higher UACR was significantly associated with higher WML volume and greater atrophy (larger cerebrospinal fluid volumes), and smaller GM and hippocampal WM volumes. A higher eGFR was associated with larger hippocampal WM volumes. Consistent with higher WML volumes, participants with CKD had significantly poorer processing speed and working memory. These findings were independent of glycemic control. CONCLUSIONS:We found albuminuria to be a better marker of cerebral structural changes than eGFR in AAs with type 2 diabetes. Relationships between albuminuria and brain pathology may contribute to poorer cognitive performance in patients with mild CKD.

BACKGROUND:Transcriptomic studies hold great potential towards understanding the human aging process. Previous transcriptomic studies have identified many genes with age-associated expression levels; however, small samples sizes and mixed cell types often make these results difficult to interpret. RESULTS:Using transcriptomic profiles in CD14+ monocytes from 1,264 participants of the Multi-Ethnic Study of Atherosclerosis (aged 55-94 years), we identified 2,704 genes differentially expressed with chronological age (false discovery rate, FDR ≤ 0.001). We further identified six networks of co-expressed genes that included prominent genes from three pathways: protein synthesis (particularly mitochondrial ribosomal genes), oxidative phosphorylation, and autophagy, with expression patterns suggesting these pathways decline with age. Expression of several chromatin remodeler and transcriptional modifier genes strongly correlated with expression of oxidative phosphorylation and ribosomal protein synthesis genes. 17% of genes with age-associated expression harbored CpG sites whose degree of methylation significantly mediated the relationship between age and gene expression (p < 0.05). Lastly, 15 genes with age-associated expression were also associated (FDR ≤ 0.01) with pulse pressure independent of chronological age. Comparing transcriptomic profiles of CD14+ monocytes to CD4+ T cells from a subset (n = 423) of the population, we identified 30 age-associated (FDR < 0.01) genes in common, while larger sets of differentially expressed genes were unique to either T cells (188 genes) or monocytes (383 genes). At the pathway level, a decline in ribosomal protein synthesis machinery gene expression with age was detectable in both cell types. CONCLUSIONS:An overall decline in expression of ribosomal protein synthesis genes with age was detected in CD14+ monocytes and CD4+ T cells, demonstrating that some patterns of aging are likely shared between different cell types. Our findings also support cell-specific effects of age on gene expression, illustrating the importance of using purified cell samples for future transcriptomic studies. Longitudinal work is required to establish the relationship between identified age-associated genes/pathways and aging-related diseases.

BACKGROUND:There is a need to identify patients with diabetic kidney disease (DKD) using noninvasive, cost-effective screening tests. Sudoscan®, a device using electrochemical skin conductance (ESC) to measure sweat gland dysfunction, is valuable for detecting peripheral neuropathy. ESC was tested for association with DKD (estimated glomerular filtration rate (eGFR) <60 ml/min/1.73 m(2)) in 383 type 2 diabetes mellitus (T2D)-affected patients; diagnostic thresholds were determined in 540 patients. METHODS:Relationships between ESC with eGFR and urine albumin:creatinine ratio (UACR) were assessed in 202 European Americans and 181 African Americans with T2D. RESULTS:In 92 European American DKD cases and 110 T2D non-nephropathy controls, respectively, mean (SD) ages were 69 (9.7) and 61 (10.8) years, hemoglobin A1c (HbA1c) 7.4 (1.2) and 7.4 (1.3)%, eGFR 29.6 (12.2) and 87.8 (14.2) ml/min/1.73 m(2), and UACR 1,214 (1,705) and 7.5 (5.8) mg/g. In 57 African American cases and 124 controls, respectively, mean (SD) ages were 64.0 (11.9) and 59.5 (9.7) years, HbA1c 7.4 (1.3) and 7.5 (1.7)%, eGFR 29.6 (13.3) and 90.2 (16.2) ml/min/1.73 m(2), and UACR 1,172 (1,564) and 7.8 (7.1) mg/g. Mean (SD) ESC (μS) was lower in cases than controls (European Americans: case/control hands 49.5 (18.5)/62.3 (16.2); feet 62.1 (17.9)/73.6 (13.8), both p < 1.3 × 10(-6); African Americans: case/control hands 39.8 (19.0)/48.5 (17.1); feet 53.2 (21.3)/63.5 (19.4), both p ≤ 0.01). Adjusting for age, sex, body mass index and HbA1c, hands and feet ESC associated with eGFR <60 ml/min/1.73 m(2) (p ≤ 7.2 × 10(-3)), UACR >30 mg/g (p ≤ 7.0 × 10(-3)), UACR >300 mg/g (p ≤ 8.1 × 10(-3)), and continuous traits eGFR and UACR (both p ≤ 5.0 × 10(-9)). HbA1c values were not useful for risk stratification. CONCLUSIONS:ESC measured using Sudoscan® is strongly associated with DKD in African Americans and European Americans. ESC is a useful screening test to identify DKD in patients with T2D.

OBJECTIVE:Relative to European Americans, African Americans manifest lower levels of computed tomography-based calcified atherosclerotic plaque (CP), a measure of subclinical cardiovascular disease (CVD). Potential relationships between CP and cerebral structure are poorly defined in the African American population. We assessed associations among glycemic control, inflammation, and CP with cerebral structure on MRI and with cognitive performance in 268 high-risk African Americans with type 2 diabetes. RESEARCH DESIGN AND METHODS/METHODS:Associations among hemoglobin A1c (HbA1c), C-reactive protein (CRP), and CP in coronary arteries, carotid arteries, and the aorta with MRI volumetric analysis (white matter volume, gray matter volume [GMV], cerebrospinal fluid volume, and white matter lesion volume) were assessed using generalized linear models adjusted for age, sex, African ancestry proportion, smoking, BMI, use of statins, HbA1c, hypertension, and prior CVD. RESULTS:Participants were 63.4% female with mean (SD) age of 59.8 years (9.2), diabetes duration of 14.5 years (7.6), HbA1c of 7.95% (1.9), estimated glomerular filtration rate of 86.6 mL/min/1.73 m(2) (24.6), and coronary artery CP mass score of 215 mg (502). In fully adjusted models, GMV was inversely associated with coronary artery CP (parameter estimate [β] -0.47 [SE 0.15], P = 0.002; carotid artery CP (β -1.92 [SE 0.62], P = 0.002; and aorta CP [β -0.10 [SE 0.03] P = 0.002), whereas HbA1c and CRP did not associate with cerebral volumes. Coronary artery CP also associated with poorer global cognitive function on the Montreal Cognitive Assessment. CONCLUSIONS:Subclinical atherosclerosis was associated with smaller GMV and poorer cognitive performance in African Americans with diabetes. Cardioprotective strategies could preserve GMV and cognitive function in high-risk African Americans with diabetes.

BACKGROUND:Despite evidence of an association between variants at the apolipoprotein L1 gene (APOL1) locus and a spectrum of related kidney diseases, underlying biological mechanisms remain unknown. An earlier preliminary study published by our group showed that an APOL1 variant (rs73885319) modified the association between high-density lipoprotein cholesterol (HDLC) and estimated glomerular filtration rate (eGFR) in African Americans. To further understand this relationship, we evaluated the interaction in two additional large cohorts of African Americans for a total of 3,592 unrelated individuals from the Howard University Family Study (HUFS), the Natural History of APOL1-Associated Nephropathy Study (NHAAN), and the Atherosclerosis Risk in Communities Study (ARIC). The association between HDLC and eGFR was determined using linear mixed models, and the interaction between rs73885319 genotype and HDLC was evaluated using a multiplicative term. RESULTS:Among individuals homozygous for the risk genotype, a strong inverse HDLC-eGFR association was observed, with a positive association in others (p for the interaction of the rs73885319 × HDLC =0.0001). The interaction was similar in HUFS and NHAAN, and attenuated in ARIC. Given that ARIC participants were older, we investigated an age effect; age was a significant modifier of the observed interaction. When older individuals were excluded, the interaction in ARIC was similar to that in the other studies. CONCLUSIONS:Based on these findings, it is clear that the relationship between HDLC and eGFR is strongly influenced by the APOL1 rs73885319 kidney risk genotype. Moreover, the degree to which this variant modifies the association may depend on the age of the individual. More detailed physiological studies are warranted to understand how rs73885319 may affect the relationship between HDLC and eGFR in individuals with and without disease and across the lifespan.

Albuminuria and reduced estimated glomerular filtration rate (eGFR) associate with two apolipoprotein L1 gene (APOL1) variants in nondiabetic African Americans (AAs). Whether APOL1 associates with subclinical atherosclerosis and survival remains unclear. To determine this, 717 African American-Diabetes Heart Study participants underwent computed tomography to determine coronary artery-, carotid artery-, and aorta-calcified atherosclerotic plaque mass scores in addition to the urine albumin:creatinine ratio (UACR), eGFR, and C-reactive protein (CRP). Associations between mass scores and APOL1 were assessed adjusting for age, gender, African ancestry, body mass index (BMI), hemoglobin A1c, smoking, hypertension, use of statins and angiotensin-converting enzyme inhibitors, albuminuria, and eGFR. Participants were 58.9% female with mean age 56.5 years, eGFR 89.5 ml/min per 1.73 m(2), UACR 169.6 mg/g, and coronary artery-, carotid artery-, and aorta-calcified plaque mass scores of 610, 171, and 5378, respectively. In fully adjusted models, APOL1 risk variants were significantly associated with lower levels of carotid artery-calcified plaque (β=-0.42, s.e. 0.18; dominant model) and marginally lower coronary artery plaque (β=-0.36, s.e. 0.21; dominant model), but not with aorta-calcified plaque, CRP, UACR, or eGFR. By the end of a mean follow-up of 5.0 years, 89 participants had died. APOL1 nephropathy risk variants were significantly associated with improved survival (hazard ratio 0.67 for one copy; 0.44 for two copies). Thus, APOL1 nephropathy variants associate with lower levels of subclinical atherosclerosis and reduced risk of death in AAs with type 2 diabetes mellitus.